A sofa bed comprises a mattress frame, a back frame and two arm frames, the mattress frame and the back frame are connected to the arm frames by hinge devices. The hinge device comprises a base, a front side connecting piece, a front support plate, a rear side connecting piece, a rear support plate, a bridge connecting piece, a spring and a spring connecting piece; the front support plate and the rear support plate are respectively fixed to the mattress frame and the back frame, the front side connecting piece is disposed between the front support plate and the base, the rear side connecting piece is disposed between the rear support plate and the base, the bridge connecting piece is disposed between the front side connecting piece and the rear side connecting piece. Existing sofa bed has technical problems: firstly, the sofa bed needs large quantity of hinge devices, the structure is complicated, the cost is high, the weight of the sofa bed is also increased. Secondly, the hinge device needs large space, therefore, to increase the container quantity, the hinge devices are packed individually before the sofa bed leaves the factory; consumers need to install the hinge devices to the arms when assembling the sofa bed, and then fix the mattress frame and the back frame respectively to the front support plate and the rear support plate, the assembly is complicated. Thirdly, many bolts are needed to lock between the base and the arm, the front support plate and the mattress frame, the rear support plate and the back frame, the bolt quantity is large, the cost is high, the weight of the sofa bed is increased, the assembly is cumbersome.